1. Field of the Invention
This invention relates to a carbon negative electrode material which is suitably used as a negative electrode of a rechargeable lithium secondary cell and also to a lithium secondary cell containing the same.
2. Description of the Related Art
Lithium primary cells employing lithium metal as negative electrodes are widely employed because of many merits. In the negative electrode of such cell, the lithium metal is oxidized into lithium ions during use (discharge) to be eluted by an organic solvent serving as an electrolyte, and free electrons formed simultaneously are supplied to an external circuit.
If such excellent performance of the lithium primary cell is tried to be utilized as such in a rechargeable secondary cell, the lithium ions are reduced by free electrons supplied from the external circuit contrariwise during discharging on the metal lithium electrode and deposit as metal lithium.
However, when metal lithium deposits on the negative electrode, it deposits in the form of granule or dendrite, inhibiting the negative electrode to resume its original form, causing various troubles including short circuits between the electrodes. Accordingly, the constitution of the lithium primary cell cannot be employed in the secondary cell in which charging and discharging are reversibly repeated.
According to recent studies, it is reported that if a carbon material is used as the negative electrode in place of metal lithium, the carbon material is reversibly doped or undoped with lithium ions, and the resulting cell can be used as a rechargeable secondary cell. Such cells are being put into practical uses. More specifically, the carbon material serving as the negative electrode is doped with lithium ions for charging and undoped for discharging. Accordingly, the amount of lithium ions doped into the carbon material is decisive of the charge capacity at the negative electrode; whereas the amount of undoped lithium ions is decisive of the discharge capacity at the negative electrode.
As the carbon material employable as the negative electrode of the lithium secondary cell, graphite was first focused upon which can be doped with lithium ions in the form of intercalation compound. In this case, one lithium atom per 6 carbon atoms is theoretically the maximum amount of doping at the negative electrode, which also decides the maximum charge capacity.
Accordingly, in order to improve cell capacity of a lithium secondary cell, it is necessary to improve charge capacity of the carbon material used as the negative electrode and also to allow the discharge capacity to approximate to the charge capacity so as to minimize irreversible capacity.
Under such circumstances, it has recently been reported that there can be obtained cell capacity values higher than the theoretical values when graphite is used, if a hard carbon (non-graphitizable carbon) material which is a porous material having a high specific surface area is used as a carbon negative electrode material. The hard carbon material is generally produced by subjecting an organic compound containing carbon as a major component to dry distillation and then heat treatment so as to develop the structure of carbon atom arrangement. Since final characteristics of the hard carbon material is controlled by the latter heat treatment step, the cell capacity (both discharge capacity and charge capacity) is greatly dependent upon the heat treatment temperature.
Meanwhile, it is also reported that, in a relationship between the heat treatment temperature and cell capacity when a hard carbon material prepared by using a furfuryl alcohol resin raw material is used as the negative electrode of a lithium secondary cell, the charge capacity assumes a maximum value at a relatively low heat treatment temperature of 800.degree. C., and the discharge capacity assumes a maximum value at a relatively high heat treatment temperature of 1,100.degree. C. Accordingly, if such carbon material heat-treated at 800.degree. C. is used as the negative electrode, the total amount of charge cannot completely be consumed for discharging to leave some irreversible capacity. Actually, since there is used a carbon material heat-treated at 1,100.degree. C. which can provide the maximum discharge capacity, the resulting cell is put into uses as incompletely charged. That is, in the conventional lithium secondary cell employing a hard carbon material as the negative electrode, discharging efficiency (discharge capacity divided by amount of charge) decreases if the charge capacity is increased, to give a reduced amount of discharge, inconveniently.
However, there is no established theory on the mechanism of electrode reaction how the hard carbon electrode is doped and undoped with lithium ions in the lithium secondary cell. Accordingly, under the present circumstances, the irreversible capacity caused by the difference between the charge capacity value and discharge capacity value has not been elucidated yet.